The mechanical energy balance is the first thing
you use to determine a situation. If that suggests
that there is significant shear or turbulence then
you must go to the equation of motion and friction
factor charts to account momentum transfer.

Everyone is I think missing what my question is.
We all agree that there is an increase in

*kinetic*

Quote:

energy with the increased velocity.
Since the total energy can't change something
has to decrease.

It *can* change. The change is usually small.

Quote:

Bernoulli's equation (which I have no doubts
about it's validity) says that the pressre
decreases. Why doesn't the temperature of
the fluid decrease to make up for the decrease
in the velocity (instead of pressure)?

It does, but *very* slightly for Mach numbers less than 0.3.

Think "Second Law of Thermodynamics"... and what temperature
represents. In fluids, it represents a measure of entropy. In a
closed system (such as along Bernoulli's streamlines) entropy is
required to stay the same (fixed temperature) or increase
(increasing temperature). So if the streamline is closed,
entropy cannot leave the system...

The mechanical energy balance is the first thing you use to determine a
situation. If that suggests that there is significant shear or
turbulence then you must go to the equation of motion and friction
factor charts to account momentum transfer.

Bret Cahill
Everyone is I think missing what my question is. We all agree that there is

an increase in energy with the increased velocity. Since the total energy
can't change something has to decrease. Bernoulli's equation (which I have
no doubts about it's validity) says that the pressre decreases. Why doesn't
the temperature of the fluid decrease to make up for the decrease in the
velocity (instead of pressure)?

The mechanical energy balance is the first thing you use to determine a
situation. If that suggests that there is significant shear or
turbulence then you must go to the equation of motion and friction
factor charts to account momentum transfer.

It is *common* and *expected* that the flow is inviscid for
Bernoulli. This is what is presented in most literature.
Inviscid equates to "frictionless" in the "block and trolley"
world of dynamic modelling.

Along with this is the general form where "potential" equates to
"kinetic" along streamlines. No frictional loss term for
increasing temperature.

I was thinking about the Bernulli effect, where the
pressure of a fluid in motion decreases as the
velocity increases. I can understand that some
characteristic has to decrease in order to not
have an overall increase in energy: but why is it
not the temperature of the fluid?

Gas cools going through a nozzle and warms
going through a diffuser in isentropic flow.

I was thinking about the Bernulli effect, where the pressure of a fluid in
motion decreases as the velocity increases. I can understand that some
characteristic has to decrease in order to not have an overall increase in
energy: but why is it not the temperature of the fluid?

Gas cools going through a nozzle and warms going through a diffuser in
isentropic flow. The temperature can calculated using the ideal gas
law and the heat capacity ratio Cp/Cv of the gas.

I was thinking about the Bernulli effect, where the
pressure of a fluid in motion decreases as the
velocity increases. I can understand that some
characteristic has to decrease in order to not
have an overall increase in energy: but why is
it not the temperature of the fluid?

Bernoulli does not describe entropy. Temperature is a measure of
entropy.

Bernoulli describes the interchange between kinetic energy
(velocity) and potential energy (pressure). Temperature is along
the lines of frictional losses... energy that is no longer
available as either kinetic or potential.

I was thinking about the Bernulli effect, where the pressure of a fluid in
motion decreases as the velocity increases. I can understand that some
characteristic has to decrease in order to not have an overall increase in
energy: but why is it not the temperature of the fluid?